As the helmet size, generally, four sizes, i.e., S size (55 cm to 56 cm), M size (57 cm to 58 cm), L size (59 cm to 60 cm), and XL size (61 cm and more) are known. These values indicate the circumferential lengths of the heads of the helmet wearers. In the helmets of the four sizes, those portions of their head accommodating spaces which correspond to the head circumferential lengths have substantially elliptic shapes substantially similar to each other.
In helmet shops where helmets are sold, when selling a helmet to an expected helmet purchaser such as an expected helmet wearer who plans to wear a helmet, a helmet of the size informed by the expected helmet purchaser is sold to him or her. Alternatively, the circumferential length of the head of the expected helmet wearer is actually measured with a tape measure or the like. The type of the matching size is selected from the matching size displayed on the tape measure that corresponds to the actual measurement value, and a helmet of the selected type is sold.
Employment of a personal computer which uses software (i.e., a computer program) for arithmetic calculation of an elliptic shape at a helmet shop or the like has conventionally been known. In this case, the back-and-forth length and right-to-left width of the head of the expected helmet wearer are actually measured with a head size measuring tool, a measure or any other measurement tool. The circumferential length of the head is calculated from the actual measurement values by using the personal computer. The type of a recommendable helmet size is selected from the calculation value and the size marks attached to helmets of various sizes.
The conventional method as described above has the following drawbacks. More specifically, when the circumferential length of the head is to be actually measured with a tape measurement as described above, or when the circumferential length of the head is to be calculated by using a program for arithmetic calculation of an elliptic shape, it is assumed that the head of the expected helmet wearer has a specific elliptic shape substantially similar to an average elliptic shape of a human head. Hence, the ratio of the major axis to the minor axis of the specific elliptic shape is substantially constant regardless of the type of the helmet size.
The shapes of the circumferences of the heads of expected helmet wearers differ from one wearer to another even if the circumferential lengths are equal, and do not always coincide with the specific elliptic shape described above. For this reason, if the type of the helmet size is selected based on only the circumferential length of the head of the expected helmet wearer, sometimes the helmet of the size of the selected type does not fit (that is, match) the head of the expected helmet wearer. In this case, the expected helmet wearer must wear a helmet of an excessively large size.